Molecular, cellular, and clinical abnormalities in patients_with xeroderma pigmentosum (XP), with the dysplastic nevus syndrome (DNS) of familial cutaneous melanoma, and with Bloom's syndrome (BS) are being studied. We have developed new assays utilizing plasmids as tools to measure DNA repair, DNA ligation and mutagenesis at the molecular level in cultured human cells. We utilized a shuttle vector plasmid, pZ189, to determine that there is a restricted spectrum of mutations induced in ultraviolet (UV)- treated DNA replicating in XP cells of complementation groups A and D. The generation of plasmids with multiple base substitution mutations was related to an error-prone polymerase activity acting on nicked DNA which may be relevant to generation of immunoglobulin diversity. DNS cells introduced more mutations into UV-treated pZ189 than normal cells but had fewer tandem mutations. The major UV photo-product, the T-T cyclobutane dimer, was found to be only weakly mutagenic in XP, DNS and normal lines. We determined that photoproduct frequency was not the major determinant of UV mutation frequency in plasmids replicated in human cells. BS cell lines were reported by others to have diminished ligase activity in vitro. Utilizing a linearized replicating plasmid we demonstrated reduced ability of BS cells to ligate plasmids in vivo. The BS cells also introduced more deletion mutations than normal cells into the recircularized plasmids. A Registry of XP patients has been established. Utilizing an assay of G2 phase chromosomal hypersensitivity, collaborative studies have, for the first time, detected XP heterozygotes. A 3-year clinical trial of cancer chemoprevention demonstrated that high dose (2 mg/kg/da) oral 13- cis retinoic acid (Accutane) is effective in preventing formation of new skin cancers in patients with XP.